This summer, Isabella Boylston, the American Ballet Theater principal, will realize a dream, to bring ballet to her hometown, Sun Valley, Idaho. Ballet Sun Valley, a three-day event with performances Aug. 22 and 24 at the Sun Valley Pavilion, and free classes for children on Aug. 23, will feature dancers from major companies, including Ballet Theater, New York City Ballet, the Royal Ballet, San Francisco Ballet and the Mariinsky Ballet.